In recent years, scaling-down based on Moore's law is facing limits and manufacturing cost of semiconductor devices has increased. Hence, it is considered to be very effective to stack elements within silicon serving as a semiconductor device's substrate, and utilize them to increase the number of elements and functions per unit volume, and the like.
For example. Patent Literature 1 discloses an image sensor with enhanced function in the stacking direction by stacking silicon by epitaxial growth where a crystalline layer with aligned crystal axes is grown on a substrate, after forming a structure on a silicon surface. Patent Literature 1 also proposes forming a mark by using a structure made of a material other than silicon. When a material other than silicon is used, however, lattice mismatch between the material and silicon causes a defect during epitaxial growth; hence, a level difference formed by etching silicon is desirably used as a mark.
Patent Literature 2 discloses a solid-state image sensor that includes a plurality of semiconductor layers formed by stacking silicon by epitaxial growth and forming a P-type well region once again after forming a P-type well region on a silicon surface by ion implantation.
Under low-temperature epitaxial conditions effective for suppressing auto-doping and reducing diffusion of a layer formed by ion implantation, a mark distortion tends to increase. Hence, Patent Literature 2 proposes stacking two layers of silicon under two types of conditions, namely an epitaxial condition causing the mark distortion while providing optimal characteristics, and an epitaxial condition sacrificing characteristics while exhibiting little influence of mark distortion. However, such a technology of performing epitaxial growth in two layers is a compromise of mark distortion, and characteristics and quality. It is originally desirable to allow precise detection of marks under epitaxial conditions that are advantageous in characteristics and quality.
In addition, Patent Literature 3 discloses a solid-state image sensor in which silicon includes a stacked pixel structure. Furthermore, with regard to bipolar devices, power devices, and the like, stacked semiconductor devices formed by epitaxial growth have been manufactured.
Further, Patent Literature 4 discloses a technology of, after a step of performing epitaxial growth, soaking a distorted shape (cross section) in a potassium hydroxide (KOH) solution and etching a crystal plane of silicon to enhance the mark's contrast. However, when the mark after the epitaxial growth is additionally processed, the processing itself causes variation of mark shapes; hence, it is desirable to avoid increasing the number of processing steps for the mark, as much as possible.